Unity 5 seems to can't find my Android Plugin

So, I wanted to create an Android Plugin for Unity to use Bluetooth and here is what I did:

• Crated an Android Project in Eclipse with minimum API 8, target API 21 and compile with API 22

• Wrote this class

 package com.example.pluginabau;
 
 import java.util.HashMap;
 import java.util.Set;
 
 import android.bluetooth.BluetoothAdapter;
 import android.bluetooth.BluetoothDevice;
 
 public class DeviceHandler {
     
     private BluetoothAdapter myBluetoothAdapter;
     
     private HashMap<String, RobotDevice> pairedDevices = new HashMap<String, RobotDevice>();
     
     public DeviceHandler() 
     {
         myBluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
         Set<BluetoothDevice> tmp = myBluetoothAdapter.getBondedDevices();
         
         for (BluetoothDevice device : tmp)
         {
             if (device.getName().startsWith("HC")) 
             {
                 pairedDevices.put(device.getAddress(), new RobotDevice(device));
             }
         }
         
         connectBT();
     }
     
     public void connectBT() 
     {
         
         if (!myBluetoothAdapter.isEnabled()) 
         {
              myBluetoothAdapter.enable();
         } 
         
     }
     
     public String prova() {
         String str = "";
         for (BluetoothDevice device : myBluetoothAdapter.getBondedDevices()) {
             str += device.getName() + 0;
         }
         return str;
     }
     
 }

• Checked the project as a library (I also had to include "appcompat_v7" as well.

• Exported the project as .jar

• In Unity 5 created the folders Assets->Plugins->Android and put the .jar into Android folder.

• Set Project Settings->Editor->Device to Any Android Device

• Created this C# script

     using UnityEngine;
 using System.Collections;
 using System.Runtime.InteropServices;

 public class Prova : MonoBehaviour {


     // Use this for initialization
     void Start () {
         AndroidJavaObject jo = new AndroidJavaObject("com.example.pluginabau.DeviceHandler"); 
         string myText= jo.Call<string>("prova"); 
         TextMesh mesh= GameObject.Find("New Text").GetComponent<TextMesh>();
         
         mesh.text = myText;
     
     }

     // Update is called once per frame
     void Update () {
     
     }
 }

I simply want my 3D Text object set to the value returned by the java method "prova". I'm also using Unity Remote 4 on my Nexus 5 (Android version 5.1.1) for testing porpuse, but everytime i press Play i get this error on the creation of the AndroidJavaObject:

Exception: JNI: Init'd AndroidJavaClass with null ptr!

UnityEngine.AndroidJavaClass..ctor (IntPtr jclass) (at C:/buildslave/unity/build/Runtime/Export/AndroidJavaImpl.cs:539)

UnityEngine.AndroidJavaObject.get_JavaLangClass () (at C:/buildslave/unity/build/Runtime/Export/AndroidJavaImpl.cs:517)

UnityEngine.AndroidJavaObject.FindClass (System.String name) (at C:/buildslave/unity/build/Runtime/Export/AndroidJavaImpl.cs:508)

UnityEngine.AndroidJavaObject._AndroidJavaObject (System.String className, System.Object[] args) (at C:/buildslave/unity/build/Runtime/Export/AndroidJavaImpl.cs:110)

UnityEngine.AndroidJavaObject..ctor (System.String className, System.Object[] args) (at C:/buildslave/unity/build/artifacts/generated/common/runtime/AndroidJava.gen.cs:17)

Other things that may be useful to know:

• USB Debugging and Allow Mock Locations options are enabled

• Device is correctly recognized (checked on Eclipse and with prompt command "adb devices")

• I also tried to build the apk and to run it directly on the device, but nothing changes

• The project was originally created on Unity 4.x and upgraded when I opened it with Unity 5.

Any ideas on what the problem can be?

Thank you very much in advance